Field of the Invention
The present invention relates to a method for reselecting, by user equipment, a cell in a mobile communication system and user equipment using the same.
Related Art
With the advent of a new device, such as a smart phone, the amount of traffic required for a wireless communication system is explosively increased. The same is true of recently deployed long term evolution (LTE) carriers.
A common solution for handling increasing traffic, in particular, a traffic increase in a specific area called a hot spot is to use multiple carriers, that is, a plurality of carriers.
Load balancing is important when multiple carriers are used. That is, it is preferred to properly distribute loading to carriers in terms of performance improvement of a system and resource efficiency. In order to achieve load balancing for multiple carriers, various deployment scenarios according to the capacity of each carrier and the number of carriers given to a corresponding area need to be taken into consideration.
If load balancing is achieved, available frequency resources can be optimally used. Multiple carriers are spaced apart from each other in a frequency domain, the band of each carrier is different, and a different number of carriers having a different band may be used in a different area. Load balancing is important in order to efficiently use available frequency resources between such carriers.
Meanwhile, it has been known that active traffic loading and access loading are closely related to the density of UE in the idle state.
Research of current traffic models shows that a ratio of the number of active user equipment and the number of user equipment in the idle state has been a statically stabilized value. Access loading is directly related to the density of user equipment in the idle state. That is, access loading is a good metric indicative of the density of user equipment in the idle state. Access loading may be used to measure the density of user equipment in the idle state and to redistribute pieces of user equipment in the idle state.
When overload (overloading) of active traffic loading and/or access loading is present, if only access barring or active traffic redirection is performed, an overload state may continue because more user equipment in the idle state may wake up.
Furthermore, overload control may need to continue in order to perform access barring and active traffic redirection. As a result, more resources are wasted for an overload control operation, more service delay is generated, and a service stop may also be increased.
Accordingly, before overload of active traffic loading or access loading is generated, it is preferred that pieces of user equipment in the idle state are offensively redistributed.
If load balancing depends on only handover or the redirection of loading after a call is set up, more handover or more redirection of loading may be required. Accordingly, handover or redirection to another frequency for each piece of user equipment increases latency and uncertainty.
The redirection of load for active user equipment may cause handover latency because it increases signaling overhead for the user equipment or may cause a handover failure due to the limit of control channel resources.
Accordingly, the generation of overload of active traffic loading or access loading can be minimized by offensively redistributing pieces of user equipment in the idle state.
That is, for load balancing, when pieces of user equipment in the idle state reselect cells, carriers need to be properly selected so that loading is equally distributed for each carrier (i.e., frequency).
In a current cell reselection method, however, when user equipment in the idle state reselects a cell, a priority value for each carrier (frequency) is applied. A current carrier-based priority-based cell reselection method has only an on/off type load redistribution effect in carriers having different priorities.
A redistribution method for user equipment in the idle state is required for load balancing for each carrier in an area in which multiple carriers are supported. That is, there is a need for a cell reselection method for achieving load balancing in an area to which multiple carriers are applied, such as a hot spot, and user equipment using such a method.